In the fabrication of semiconductor wafers, the photolithography steps are critical when patterning the minimum feature width, dictated by given photolithography equipment, onto a wafer. Several factor come into play that will affect the dimension and profile of a structure that has resulted from the photolithography steps performed.
One factor is the quality of the masking material (photoresist) itself. Another factor is the effectiveness of the light source (usually ultraviolet light) to expose the photoresist in direct correlation to an overlying mask or reticle. Though, the photoresist quality must continually be monitored and improvements made, the exposure of the photoresist to a light source to provide the desired patterned, is an area where major engineering efforts are ongoing.
The effectiveness of proper light exposure of the photoresist, not only depends on the photoresist material itself, but also on other factors such as the type of underlying film that is being patterned. Maintaining a desired profile becomes even more difficult when patterning a material having a rough surface, such as a refractory metal that is made rough by the shape of its grains.
When patterning a refractory metal, the unevenness of the grains results in the film possessing non-uniform adsorption and reflective properties to light. During a photo step, these non-uniform properties to light result in the light reflecting back into the photoresist at varying angles to cause reflective notching of the photoresist. Though reflective notching can be caused by any underlying film that is being patterned, it is a major problem when patterning the rough surfaced refractory metal.
Another challenge that is presented by the uneven grain of a refractory metal presents, comes to play during the etching step. Usually it is desired to obtain the most vertical profile as possible. However, the uneven grains of the refractory metal silicide, cause the vertical profile to become jagged and less vertical, both undesirable results.
What is needed is a method that addresses the patterning and etching of a refractory metal to obtain structures having a substantially vertical profile. The present invention teaches such a method that may be used in semiconductor processes, such as fabrication processes for Dynamic Random Access Memory (DRAM), Static Random Access Memory (SRAM) and the like.